1. Field of the Invention
The present invention relates to a piston for an internal combustion engine.
2. Description of the Prior Art
Various structures for lightening a piston and decreasing friction between a piston and a cylinder bore have been proposed to thereby improve the fuel economy and power of the engine. For example, Japanese Patent Publication SHO 61-58954 proposes a piston wherein a ratio of an axial length of a piston skirt to a cylinder bore diameter is selected within the range of 0.15-0.25 to thereby conspicuously shorten the skirt length, to lighten the piston and to decrease the piston friction.
However, the prior art piston suffers from the following problems.
First, selection of the skirt length only will not necessarily decrease the friction between the piston and the cylinder bore, because the piston friction depends on the skirt bearing pressure (namely, the thrust/counter-thrust force/the skirt surface area), which in turn depends not only on the skirt length but also depends on the skirt width.
Second, if the ratio of the skirt length to the bore diameter is set in the above-described range, the lower end of the skirt will come to a position above the lowermost portion of the piston-pin hole. This makes the piston attitude unstable in the thrust/counter-thrust direction.
Therefore, the above-described structure is not necessarily optimal from the viewpoints of piston friction and piston attitude.